There is a threading device in which compressed air is sprayed onto a thread inserting port of a thread pipe, guiding a lower thread toward a thread hole of a looper, to insert the lower thread into the thread hole of the looper through the thread pipe by an air flow (see, e.g., Japanese Patent No, 2865470).
When the looper, which swivels back and forth, is at a front position, an upper thread is usually being caught by the looper. When inserting the lower thread into the thread hole of the looper in this state, it is cumbersome to pull away the upper thread passing through the looper or to consider the order of threading.
In a looper mechanism disclosed in the Japanese Patent No. 2865470, moreover, the looper is swiveled interlockingly with a rotation of a main shaft (a lower shaft) which serves as a driving shaft. Thus, in order to prevent the threading work from being hindered by a movement of the looper, it is necessary to stop and keep the looper at a predetermined position when inserting the lower thread into the looper.
Accordingly, as shown in FIG. 14, an operator of the sewing machine has to manually operate a locking mechanism for locking the main shaft by one hand while rotating the main shaft so as to find a phase of the looper (i.e., a rotating angle of the main shaft) that matches the locking mechanism by the other hand. In other words, when inserting the lower thread into the thread hole of the looper, it is necessary to use both hands. Thus, there is a problem that the operations are cumbersome.